1. Field of the Invention
The invention disclosed in this specification relates to the structure of thin film semiconductor element such as thin film transistor and thin film diode and to the production thereof, and also to the thin film semiconductor to construct these thin film semiconductor elements and to the production thereof.
2. Description of the Prior Art
Recently, optical apparatus such as liquid-crystal display apparatus and image sensor that use thin film transistors are being developed. This uses a glass substrate as the substrate, forms a thin film semiconductor on the surface of this glass substrate, constitutes an active layer of thin film transistor using this thin film semiconductor, and constitutes the drive circuits and switching circuits of optical apparatus using this thin film transistor.
As the thin film silicon semiconductor formed on an insulating substrate such as glass, the technique that uses a silicon film having crystal properties is known. The transistor that uses this thin film silicon semiconductor having crystal properties is expected to have high characteristics and the application to liquid crystal display devices of active matrix type and thin film integrated circuits is expected.
FIG. 1 shows one example of the fabricating process of the thin film transistor utilizing the crystalline silicon film.
First, a silicon oxide film 102 as an underlying film is formed to a thickness of 3000 .ANG. on a glass substrate 101. Then, an amorphous silicon film 103 is formed thereon to a thickness of 200 to 1000 .ANG. by the plasma CVD method.
And the amorphous silicon film 103 is crystallized by conducting heating at 600.degree. C. and irradiation with laser light. The reason why the heating temperature is specified at 600.degree. C. is that, in general, the glass substrate presents difficulties in applying heat treatment at temperatures above 600.degree. C.
However, in order to crystallize the amorphous silicon film 103 by heating and to obtain a crystalline silicon film having good crystal properties (having crystal properties good enough for practical use), it is necessary to apply heat treatment at temperatures above 800.degree. C., preferably above 1000.degree. C.
Thus, the technique using irradiation with laser light in combination, thereby improving the crystal properties, no matter what the improvement, is utilized. Incidentally, it is possible to crystallize the amorphous silicon film 103 only by irradiation with laser light, but this is not practical because when the amorphous silicon film 103 is crystallized only by irradiation with laser light, there is a problem in the uniformity of the film quality thus obtained.
That is, in the case where one attempts to irradiate laser light over a large area at a time, the unevenness of laser power strength in the beam pattern becomes a problem and it becomes difficult to irradiate laser light at uniform power over the entire surface. Also, there is a method of crystallizing the amorphous silicon film 103 while scanning laser light; however, in this case there is a problem that the irradiating power of laser light gradually changes with the lapse of time. Likewise, it is difficult to irradiate the laser light uniformly over a large area.
These problems are solved by using with a margin a laser oscillating apparatus having a large irradiating power; however, in this case, there occurs another problem that the cost required to produce and maintain the laser oscillating apparatus becomes high.
Thus, improving the uniformity of quality of the resulting crystalline silicon film and productivity through crystallization by heating and laser light irradiation is being practiced.
After crystallization of the amorphous silicon film 103 in the step shown in FIG. 1(A), the active layer 104 of thin film transistor is formed by patterning. And, the silicon oxide film 105 to become the gate insulating film is formed to a thickness of 1000 .ANG. by the plasma CVD method. Moreover, the gate electrode 106 is formed from metal or silicide or even from crystalline silicon semiconductor having one conductivity type. (FIG. 1 (B))
After the state shown in FIG. 1(B) has been obtained, the implantation of impurity ions (such as phosphorus ions) to impart one conductivity type is carried out, thereby forming the source/drain regions in self-aligned manner. In this case, the gate electrode 106 becomes a mask so that impurity ions are implanted only into the region to become the source region 108 and the region 112 to become the drain region. The channel forming region 110 is also formed in self-alignment manner. This step is called the self-alignment process. (FIG. 1(C))
After the implantation of impurity ions has been conducted in the step of FIG. 1(C), irradiation of laser light is carried out, thereby performing the recrystallization of the source/drain regions 108, 112 and the activation of implanted ions. The reason why laser light irradiation is necessary is that the region into which impurity ions have been implanted is made amorphous by the shock of implanted ions, and the implanted ions as such are not yet activated as required of silicon for one conductivity type.
Then, the silicon oxide film 113 as the interlayer insulating film is formed to a thickness of 7000 .ANG. by the plasma CVD method. Contact holes are formed in the silicon oxide film 113, and the source electrode 114 and the drain electrode 115 are formed there. In this way the thin film transistor is completed. (FIG. 1(D))
The thin film transistor using crystalline silicon film formed on the glass substrate as shown in FIG. 1 is such that the off current characteristics is a serious problem. The off current denotes the current that flows between the source/drain regions when the thin film transistor is in the off state.
For example, when a thin film transistor is arranged in each picture element of a liquid crystal display device of active matrix type, the thin film transistor arranged in each picture element retains charge in the picture element electrode and hence functions as a switch, but if the off current is great, the charge held in the picture element escapes and it is impossible to maintain the image data for a length of time required.
This problem of off current is a serious problem in the case where the memory is constructed from thin film transistors using crystalline silicon film, and hence there is a demand for its improvement.
In addition, the crystalline silicon film formed as mentioned above has the polycrystalline structure or microcrystalline structure, and from the viewpoint of crystal, it is not necessarily good in crystal properties in many cases, and the amorphous component and trap level are present in the film in a nonnegligible density. Such amorphous component and trap level are the major factor of the change with time (fluctuation of threshold value and deterioration of characteristics) of the characteristic properties of the thin film transistor.